A high intensity discharge lamp (HID) is used, for example, in a light source apparatus for use in an optical apparatus for displaying an image such as a liquid crystal projector or a DLP (trademark) projector.
In such a projector, a given original image is separated into three primary color components of R, G, and B by using a dichroic prism or the like, and respective three primary color images are generated by using spatial modulation devices provided for the respective three primary colors. The resultant image paths of respective three primary colors are combined by using a dichroic prism or the like again so that a single full color image may be displayed.
On the other hand, there is a projector, in which light emitted from a light source is passed through a combined three primary colors R, G, and B filter which is rotating, thereby sequentially generating light rays of three primary colors R, G, and B. In synchronization with the generation of the light rays, a spatial modulation device is controlled so as to sequentially generate respective three primary colors thereby displaying a color image.
To start such a lamp while a voltage called a no-load open-circuit voltage is applied to the lamp, a high voltage is applied thereto to cause an electrical breakdown in a discharge space, and discharge first occurs in a glow discharge mode and is then shifted to an arc discharge mode.
As a method of applying the high voltage to the lamp, there is an external triggering method in which an auxiliary electrode is provided in addition to the main discharge electrodes such that the auxiliary electrode is not in contact with a discharge space, and a high voltage is applied to the auxiliary electrode, in addition to a serial triggering method in which the high voltage is superimposed to main discharge electrodes by using an igniter.
The external triggering method has various advantages over the serial triggering method. In particular, where a high voltage generator including a high voltage transformer is separated from a power supply circuit and is disposed close to a discharge lamp, the external triggering method is particularly advantageous such as a reduction in size/weight of the discharge lamp lighting apparatus, a reduction in noise, an improvement in safety, and a reduction in cost.
As driving methods of a discharge lamp in a steady lighting state, there are a DC driving method and an AC driving method. The DC driving method has a great advantage that this method can be applied to both types of projectors since the intensity of light emitted from the lamp does not change according to time. On the other hand, the AC driving method has an advantage over the DC driving method in that there is a possibility that ablation or growth of electrodes of the discharge lamp can be controlled by using a polarity inverting frequency which does not exist in the DC driving method, although the AC driving method also has disadvantages attributing to the polarity inversion.
In general, there is a short break, an overshoot, or an oscillation of light flux emitted from the lamp each time the polarity is inverted to generate the AC voltage by which the lamp is driven. Therefore, if the AC driving method is applied to the above-described type of projector that sequentially generates color image frames in a time dividing manner, the displayed image fluctuates at a beat frequency corresponding to a difference between the timing of generating color image frames and the timing of inverting the polarity to generate the AC voltage by which the lamp is driven. Depending on the beat frequency, the fluctuation deteriorates the quality of the displayed image. Although to prevent the above problem, the polarity of the inverter is usually inverted in synchronization with the rotation of the color filter, the discharge lamp lighting apparatus becomes complicated therefore.
In the case of the DLP-type projector, since the luminance of each color of each pixel of a displayed image is controlled by the duty cycle of an operation of each pixel of the spatial modulation device, when this type of projector is driven by the AC driving method, if the overshoot or the oscillation of the light flux continues for a long period each time the polarity is inverted, it is desirable not only to control the synchronization of timing in the above-described manner but also to control the operation such that the light flux is not used during this period or such that the fluctuation is cancelled out by controlling the operation of each pixel of the spatial modulation device. However, if the light flux is not used during the above-described period, the efficiency of using the light flux is reduced. On the other hand, in the case where the operation of each pixel of the spatial modulation device of the projector is controlled in the above-described manner, very complicated control is necessary.
The disadvantages of the AC driving of the discharge lamp can be overcome by reducing the fluctuation of the light that occurs at polarity transitions. However, this is not easy to achieve, because, in the discharge lamp lighting apparatus, in addition to the reduction in the fluctuation of the light that occurs when the polarity of the voltage applied to the lamp is inverted, high reliability in certainly starting discharge of the discharge lamp is demanded.
It is known that when an electrical breakdown is created in the discharge space by applying the high voltage in the serial triggering method or the external triggering method, in order to achieve high reliability in starting of discharge in the discharge lamp, it is effective to use a high no-load open-circuit voltage applied to the lamp. In the case of the AC driving method, it is effective to increase the no-load open-circuit voltage applied to the lamp. In the AC driving method, in order to achieve the high reliability, a resonance assist is carried out, in which while a high voltage is increased by generating a series resonance to the lamp, an igniter is operated to create an electrical breakdown in the discharge space.
In FIG. 14, the principle of the resonance assist technique using the series resonance is described below.
In this example shown in FIG. 14, a discharge lamp lighting apparatus includes a power supply circuit (Ux′) for supplying electrical power to a discharge lamp (Ld), a full-bridge inverter (Ui′) comprising switch elements (Q1′, Q2′, Q3′, and Q4′) for inverting the polarity of the output voltage of the power supply circuit (Ux′), a resonant coil (Lr), a resonant capacitor (Cr), and a starter circuit (Ut″). In a starting operation, the inverter (Ui′) is operated so as to invert the polarity of the output voltage of the power supply circuit (Ux′) at a frequency equal to or close to the resonant frequency determined by the product of the inductance of the resonant coil (Lr) and the capacitance of the resonant capacitor (Cr), so that LC series resonance occurs and a high voltage is generated between two terminals of the resonant capacitor (Cr). This high voltage is applied to the discharge lamp (Ld) and the starter circuit (Ut″), connected in parallel to the resonant capacitor (Cr).
However, in such a discharge lamp lighting apparatus using the LC series resonance, although high reliability can be achieved in the lamp starting operation, it is difficult to reduce the fluctuation of the intensity of the light flux that occurs when the polarity of the voltage applied to the lamp is inverted, because of the reasons described below.
As described above, since the LC resonant frequency is determined by the product of the inductance of the resonant coil (Lr) and the capacitance of the resonant capacitor (Cr), when it is desirable to reduce the inductance of the resonant coil (Lr), it is necessary to increase the capacitance of the resonant capacitor (Cr). If both the inductance of the resonant coil (Lr) and the capacitance of the resonant capacitor (Cr) are reduced, the resonant frequency becomes too high to properly operate the inverter (Ui′). However, in order to obtain a sufficiently high enough voltage by resonance using the resonant capacitor (Cr) having capacitance set to a large value, it is required to pass a very large current through the series connection of the resonant coil (Lr) and the resonant capacitor (Cr). That is, the increase in the capacitance of the resonant capacitor (Cr) creates a problem that the resonance current becomes very large.
For example, when the switch element (Q1′) and the switch element (Q3′) are in an on-state, the resonance current flows across the whole circuit including the power supply circuit (Ux′) and the inverter (Ui′) along a path (L01) shown within a broken line of FIG. 14. Therefore, each circuit element must be designed to allow the large resonance current to pass therethrough. However, this causes an increase in the apparatus size and an increase in cost.
When the operation is performed in a high-order resonance mode although the resonant frequency is set to be very high, the inverter (Ui′) is allowed to operate at a low frequency, and the resonant capacitor (Cr) is allowed to have low capacitance. However, even in this case, the resonance current also flows through the path (L01) shown in the broken of FIG. 14, and a relatively large on-resistance of each switch element causes the resonant circuit to have a small Q value, thereby resulting in large attenuation of resonance. This makes it difficult to use the high-order resonance mode.
As described above, as long as the LC series resonance is used, it is difficult to reduce the inductance of the resonant coil (Lr), that is, it is necessary to set the inductance of the resonant coil (Lr) to a high value. On the other hand, in an operation phase in which light emitted from the lamp is used in a steady lighting state after the lamp is started, the large inductance of the resonant coil (Lr) causes a great disadvantage in the operation. More specifically, the large inductance of the resonant coil (Lr) causes an increase in an undesirable phenomenon such as an overshoot or a vibration of the light flux at polarity transitions, and thus there is still the unsolved problem that the light flux fluctuates when the polarity of the voltage applied to the lamp is inverted. See Japanese Laid Open Patent Nos. H03-030291, 2003-217888, and 2004-327117.